The basic components of electrical capacitors include conductive electrodes connected to an electric power supply and a dielectric material separating the electrodes. Electrolytic capacitors and electrochemical double layer capacitors also have an electrolyte. In an electrolytic capacitor, the dielectric is provided by an oxide layer formed on a metal foil and the electrolyte provides electrical contact to the opposite electrode. The inherently high resistance of electrolytic capacitors is generally mitigated by rolling a large sheet of the material into a roll. In an electrochemical double layer capacitor, the dielectric is provided by the electrolyte. In this type of capacitor, the resistance of the electrolyte is a significant factor in the total device resistance. In capacitors that use electrolytes, the electrolyte also has a major influence on the temperature performance of the capacitor.
Electrochemical double layer capacitors capable of high energy density, known as "supercapacitors," have been assembled from a variety of materials. In general, it is desirable to construct supercapacitors with light-weight materials and electrolytes that are stable and nonreactive, as described in U.S. Pat. No. 5,260,855 issued to Kaschmitter et al., the teachings of which are hereby incorporated by reference. This type of supercapacitor incorporates electrodes based on carbon foams that may be prepared from organic gels. Several types of foams can be produced, including aerogels, xerogels, and aerogel-xerogel hybrids. These low density carbon foams are electrically conductive, dimensionally stable, and machinable. Capacitors based on carbon foam electrodes are capable of delivering very high specific capacitance and very high power.
Electrochemical double layer capacitors, including supercapacitors, typically comprise electrodes, electrical contacts to a power supply, separators for electrodes and/or cells, an electrolyte, and environmental seals. As mentioned above, a key component of electrolytic and electrochemical double layer capacitors is the electrolyte, which typically comprises a combination of a salt and a solvent. Desirable electrolytes are typically liquid with low viscosity, low density, and high conductivity over a range of ambient temperature conditions. They should also be commercially inexpensive, chemically and electrochemically stable, and compatible with carbon. Aqueous electrolyte systems have been used extensively, but certain organic liquid systems are less prone to form gas and can be more effective in providing higher energy densities over a wider usable range of temperature and potential. A need exists for improved electrolyte systems that provide optimum capacitance for capacitors to achieve high power density, high energy density, a wide temperature range, and a long lifetime without memory effects.